Data Model: Make MQTT settings optional

To allow running the data logger without configuring them.

CHANGES.md

@@ -5,6 +5,8 @@
   `version`. Thanks, @bee-mois.
 - Emit user admonition on `make-config` subcommand. Thanks,
   @bee-mois.
+- Data Model: Make MQTT settings optional, to allow running the
+  data logger without configuring them.
 
 ## v0.0.3 - 2024-04-20
 - Tests: Make sensor tests work, using a fake sysfs filesystem

ds18b20_datalogger/model.py

@@ -66,16 +66,19 @@ class Settings:
     General settings container.
     """
 
-    mqtt: MqttSettings
-    devicemap: DeviceMap
+    mqtt: t.Union[MqttSettings, None]
+    devicemap: t.Union[DeviceMap, None]
 
     @classmethod
     def from_file(cls, configfile: Path):
         data = yaml.safe_load(configfile.read_text())
         devicemap = DeviceMap()
         for item in data["one-wire"]:
             devicemap.devices.append(Device(**item))
-        return cls(mqtt=MqttSettings(**data["mqtt"]), devicemap=devicemap)
+        mqtt = None
+        if "mqtt" in data:
+            mqtt = MqttSettings(**data["mqtt"])
+        return cls(mqtt=mqtt, devicemap=devicemap)
 
 
 @dataclasses.dataclass